1. Field of the Invention
The present invent ion relates to a control apparatus for controlling the pulley thrust control pressure which produces a thrust force to axially squeeze the belt between the sides of the pulley so as to adjust the allowable transmitting torque through the belt.
2. Relating Prior Arts
Belt-type continuously variable transmissions are previously well-known and have been put to practical use on automobiles and other vehicles. The belt-type continuously variable transmission comprises, for example, drive and driven pulleys each having a variable pulley width and a metallic V-belt trained around the drive and driven pulleys. The speed ratio thereof is controlled by controlling the pulley widths of the drive and driven pulleys. The drive and driven pulleys respectively have drive and driven hydraulic cylinders for applying axial thrust forces fending to reduce the widths of the drive and driven pulleys, respectively, which forces are produced by a pulley thrust hydraulic pressure supplied to the cylinders. The control of the pulley thrust pressures to the respective cylinders causes variations in the width of each pulley to vary the diameters of circles around which the V-belt is trained on the pulleys for controlling the speed ratio of the transmission.
The pulley thrust pressure for controlling the speed ratio is determined based on the torque transmitted through the V-belt. The torque transmission that is capable of being transmitted through the V-belt is a function of the friction between the V-belt and the pulleys. The pulley thrust pressure is so set that the transmittable torque through the V-belt, which is determined based on the friction between the V-belt and the pulleys, is larger than the actual transmitting torque through the V-belt by a certain amount. Therefore, slippages between the V-belt and the pulleys can be positively avoided to improve the transmitting efficiency.
While it is preferable to make the transmittable torque produced by the pulley thrust pressure larger than the actual transmitting torque so as to positively prevent the slippage of the V-belt, if the pulley thrust pressure is excessively high, the driving power required for the oil pump which supplies hydraulic oil for producing the pulley thrust pressure becomes excessively large to deteriorate the fuel economy of the engine driving the pump. Also, under an excessively high pulley thrust pressure, the life or durability of the V-belt may be decreased.
Accordingly, it is preferable to make the pulley thrust pressure as low as possible within an allowable range in which the slippage of the V-belt can be prevented. Several preferable methods of setting the pulley thrust pressure have previously been proposed.
One method is disclosed in Japanese Patent Publication No. 2-45062. That method comprises the steps of; calculating the engine output power based on the engine speed and the engine intake vacuum pressure, calculating a speed reduction ratio (or a speed ratio) of the transmission based on the engine speed and a driven pulley speed, and determining an optimum pulley thrust pressure based on the engine output power and the speed reduction ratio. Another method is disclosed in Japanese Patent Laid-open Publication No. 63-222943, which method comprises the steps of; calculating an engine output torque, correcting the engine output torque by subtracting the power loss inherent in the drive mechanism, and determining a pulley thrust pressure based on the corrected engine output torque. A further method is disclosed in Japanese Utility-Model Publication No. 3-38517, which method comprises the steps of; calculating an engine output torque, correcting the engine output torque by subtracting or adding an inertia torque which is produced when the pulley is accelerated or decelerated, and determining the pulley thrust pressure based on the corrected engine output torque.
The above methods can improve the pulley thrust force control. However, all of the above prior art methods are used even when the actual transmitting torque to the drive pulley is negative (that is, when an engine braking is being applied through the transmission) as well as when it is positive (that is, when the drive pulley is driven by the engine).
When the actual transmitting torque is positive, it can be accurately calculated based on the engine operating conditions. However, when the actual transmitting torque is negative, the deviation thereof is fairly large and it is difficult to accurately calculate such torque.
Therefore, in order to avoid any slippage of the V-belt even when the actual transmitting torque is negative, the pulley thrust pressure should be set at a magnitude high enough to produce a fairly high transmittable torque even under a negative transmitting torque. However, as described above, the high pulley thrust pressure may deteriorate the engine fuel economy and the V-belt life.
Moreover, continuously variable transmissions often have a starting clutch and a directional control clutch (that is, a forward-reverse clutch) that presents additional control problems. When a shift range position set by a manual shift lever is N (neutral) or P (parking) position, the clutches are released and no power is transmitted through the transmission. The above-mentioned prior methods are not suitable for the conditions when the shift range position is at N or P-position.
In order to use the above prior art methods satisfactorily under such conditions, a different control must be conducted only when the shift position is N or P. For this control, a shift position detector is necessary to detect the shift position of the transmission. A conventional position detector which is usually used with an automatic transmission to detect manual shift lever positions is often used. However, when the manual shift lever is shifted from a D-position to a L-position for example, the detection signal from the detector is changed from a D-position signal to a L-position signal through OFF-range. No signal is sent from the detector in the OFF-range. If the shift lever is accidentally held at a position between the D and L-positions (at OFF-range position), the pulley thrust pressure control may be lost or the pressure may be reduced to zero. Then the V-belt may slip to deteriorate the transmitting efficiency and the duration of the V-belt.
In overcoming the deficiencies of the prior art control methods, the present inventors have accounted for certain facts and features of this type of V-belt drive. The pulley thrust pressure is also used to control the speed ratio. To increase the speed ratio (that is, to shift to the LOW ratio), the pulley thrust pressure in the drive pulley is set to be lower than in the driven pulley to cause the effective diameter of the drive pulley to decrease and the driven pulley to increase. To decrease the speed ratio (that is, to shift to the TOP ratio), the pulley thrust pressure in the driven pulley is set to be lower than in the drive pulley to decrease the effective diameter of the driven pulley and increase the effective diameter of the drive pulley. In other words, when shifted toward LOW, the thrust pressure in the drive pulley is low and the thrust pressure in the driven pulley is high. When shifted toward TOP, the thrust pressure in the driven pulley is low and the pressure in the drive pulley is high.
It is necessary to prevent any slippage both in the drive and driven pulleys. Because slippage is more likely to occur at the pulley on which the low pulley thrust pressure is being applied, the low pulley thrust pressure is set at a level that is adequate to prevent the slippage. Therefore, the low pulley thrust pressure is determined based on the engine output torque so that the transmittable torque through the belt is higher than the actual transmitting torque by a certain amount.
The actual transmitting torque in the drive pulley is equal to the engine output torque, when it is positive. But the actual transmitting torque in the driven pulley is less than the engine output torque because of the friction loss inherent in the V-belt drive. Therefore, when the pulley thrust pressure in the driven pulley is low and is determined based on the engine output torque, the pulley thrust pressure in the driven pulley tends to be unnecessary high (by an amount corresponding to the friction loss of the V-belt).